The present disclosure relates to a memory element and a memory device storing information based on any change of electrical characteristics observed in a memory layer including an ion source layer and a resistance change layer.
In information devices such as computers, a RAM (Random Access Memory) widely in use is a DRAM (Dynamic Random Access Memory), which operates at a high speed and is high in density. The DRAM is, however, high in cost due to the complicated manufacturing process thereof compared with that of a logic circuit LSI (Large Scale Integration) or of a signal processing circuit generally used for electronic devices. The DRAM is also expected for a frequent refresh operation, i.e., an operation for reading any written information, and amplifying again the information for rewriting thereof. Moreover, as is a volatile memory from which any stored information is lost when a power supply is stopped, the DRAM has a disadvantage of not appropriate for use if long-time storage is expected.
On the other hand, a nonvolatile memory also in use is available for storage of any information even with no power supply. Such a nonvolatile memory is exemplified by FeRAM (Ferroelectric Random Access Memory) or a MRAM (Magnetic Random Access Memory). Such nonvolatile memory, however, has been pointed out that there are limitations on microfabrication considering the need for a high level of voltage for writing and erasing, and the limited number of electrons for injection to a floating gate.
For overcoming such limitations on microfabrication, a next-generation nonvolatile memory currently proposed is a memory element including an ion conductive layer interposed between two electrodes (for example, see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-536840, and Nikkei Electronics on Jan. 20, 2003 issue, pp. 104). The ion conductive layer includes metallic element, e.g., copper (Cu), silver (Ag), or zinc (Zn), and a chalcogenide element, e.g., sulfur (S), selenium (Se), or tellurium (Te). In such a memory element, one of the two electrodes includes the metal same as that in the ion conductive layer. Through application of a voltage in between the two electrodes, the metal in the electrode diffuses as ions into the ion conductive layer so that the ion conductive layer shows a change of resistance value or a change of electrical properties such as capacitance.
To be specific, in response to application of a bias voltage of a threshold value or higher to the two electrodes, the metal in the ion conductive layer is ionized, and then is moved in the direction of the negative electrode so that the metal is electrodeposited on the negative electrode. The metal electrodeposited as such grows like branches (dendrites), for example, and reaches the positive electrode. This accordingly forms a current path, and reduces the resistance value of the ion conductive layer. As such, recording of information is performed to the memory element. On the other hand, by application of a voltage opposite in polarity to the bias voltage applied to the two electrodes as above, the metal ions forming the branched current path are dissolved into the ion conductive layer. As a result, the current path disappears, and the resistance value of the ion conductive layer is restored, i.e., increased. In other words, an erasing operation of the recorded information is performed to the memory element.